(1) Field of the Invention
The present invention relates to skid landing gear and to an aircraft provided with such landing gear, in particular a rotorcraft.
(2) Description of Related Art
Conventionally, a rotorcraft includes landing gear via which the rotorcraft rests on the ground. More particularly, the landing gear may comprise so-called “skid” landing gear having first and second longitudinal bearing skids suitable for coming into contact with the ground and located on either side of the fuselage of the rotorcraft.
In addition, skid landing gear is usually provided with first and second transverse crossbars each connected to the first and second skids in order to connect the skids to the airframe of the aircraft.
The first crossbar is referred to as the “front” crossbar since the first crossbar connects the airframe to zones situated at the front ends of the first and second longitudinal skids. Conversely, the second crossbar is referred to as the “rear” crossbar insofar as the second crossbar connects the airframe to zones situated at the rear ends of the first and second longitudinal skids.
The landing gear is then fastened to the aircraft via its front and rear crossbars.
Such landing gear is very effective and enables a rotorcraft to land on multiple types of surface.
Nevertheless, landing on sloping ground can be difficult, in particular for an inexperienced pilot.
Using the landing procedure that is normally used when landing a helicopter on sloping ground, the pilot begins by hovering above the selected landing zone. The pilot then moves the helicopter towards the ground by reducing the collective pitch of the blades of the rotor until one skid touches the ground, which skid is referred to for convenience as the “uphill” skid.
The pilot then continues to reduce the collective pitch, while also modifying the cyclic pitch of the blades so as to keep the rotor disk substantially horizontal, i.e. perpendicular to the gravity axis.
It should be observed that if the slope of the ground is excessive, it is possible for the pilot to reach the maximum cyclic pitch of the blades, whereupon the pilot is obliged to take off in order to find some other landing zone.
When the downhill skid touches the ground in turn, the pilot verifies the quality of the trim of the helicopter on the ground by gently operating the rudder bar and the collective pitch lever. If the trim is acceptable, then the pilot ceases to act on the cyclic pitch of the blades and possibly reduces the speed of rotation of the power plant driving the rotor. In certain situations such as unloading passengers, it is advisable not to reduce the speed of rotation.
It can be understood that such a procedure requires special training for pilots.
Document U.S. Pat. No. 3,857,533 describes a helicopter provided with skid landing gear that makes it easier to land on sloping ground.
That landing gear possesses first and second skids that are connected together by front and rear crossbars. Each crossbar forms a circular arc with its concave side facing towards the ground, the concave surface of each of the crossbars including blocking catches.
The landing gear also includes guide means fastened to the helicopter airframe for co-operating with each of the crossbars, i.e. stationary wheels that allow the crossbars to move transversely when one of the skids touches the ground, with the wheels then revolving.
Finally, the landing gear includes active means for blocking the crossbars, i.e. fingers controlled by the collective pitch control of the aircraft so as to co-operate with the blocking catches of the crossbars.
Thus, when the helicopter lands on a slope, as soon as the uphill skid of the landing gear touches the ground, the reaction of the ground on the uphill skid causes the skids to move transversely. Under such circumstances, the landing gear pivots relative to the airframe of the helicopter.
Such pivoting comes to an end when the downhill skid touches the ground, in turn. Thus, the landing gear pivots to match the slope without the airframe of the helicopter tending to pivot as well, thus enabling the pilot to avoid acting on the cyclic pitch of the blades for the purpose of keeping the lift rotor horizontal.
Furthermore, by reducing the collective pitch of the blades with the collective stick lever, the pilot instructs the blocking means to engage in the blocking catches so as to prevent the airframe tilting sideways on the ground by freezing the position of the airframe relative to the landing gear.
Since the rotor is kept horizontal without acting on the cyclic pitch, landing is simplified and made safer compared with the above-described procedure.
Although effective, that landing gear requires the flight control to be coupled with the blocking means in order to block the landing gear on the ground. This results in a mechanical structure that is complicated and also in providing actuators and sensors, thereby running the risk of them failing.
Furthermore, it is found that after landing on sloping ground, the landing gear is off-center relative to the airframe of the helicopter and in particular relative to the anteroposterior plane of symmetry of the airframe. This results in unbalance of the aerodynamic drag of the landing gear during the flight undertaken after landing on sloping ground.
Furthermore, the pilot has no means for estimating the position of the landing gear relative to the airframe, and in particular the heights of the skids relative to the airframe. Under such circumstances, the pilot is in a disagreeable situation next time a landing is to be made. Furthermore, it can be understood that the pilot runs the risk of catching on an obstacle while moving in translation at very low altitude.
It should also be observed that the following documents are known: SE 118 333 C, FR 2 895 368 A1, and U.S. Pat. No. 3,173,632 A.